The present invention generally relates to the field of radio telecommunications. However, the present invention more particularly relates to downlink power level control in a time division multiple access (TDMA) based radio telecommunications system.
In a radio telecommunications system, such as a cellular radio telecommunications system, any one of several access strategies may be employed, for example, frequency division multiple access (FDMA), code division multiple access (CDMA), or time division multiple access (TDMA). In an FDMA based system, the frequency spectrum is divided into a number of disjunctive frequency bands, where each band serves as a separate radio channel. In a system that employs CDMA, different modulation or spreading codes are used to distinguish the various radio channels.
In a TDMA based system, however, the time domain is divided into time frames. Each time frame is then further divided into a number of timeslots. Thus, each carrier frequency-timeslot combination constitutes a different physical channel, over which, a communications signal burst can be transmitted. In a cellular radio telecommunications system, a communications signal burst transmitted from a mobile station to a corresponding radio base station is referred to as an uplink burst. In contrast, a communications signal burst transmitted from the radio base station to the mobile station is referred to as a downlink burst.
In regard to the present invention, power control refers to the ability to modify or adjust the power levels associated with communications signal bursts, particularly, downlink communications signal bursts. Although the ability to modify or adjust power level is important because it helps to insure that the signal quality associated with a given channel is adequate, the ability to modify or adjust power level is, more generally, important because it helps to improve the spectral efficiency of the system as a whole by balancing average, system-wide signal quality and system capacity.
In accordance with the interim standards for TDMA IS-136A and IS-136B, the transmission power level associated with a downlink burst must remain constant during a given time frame. Thus, a mobile station receiving a downlink burst during a given timeslot expects the power level of the received burst to be constant, or nearly constant, over the timeslot. Despite this, the power level of the received downlink burst is not always constant.
In actuality, mobile radio signal transmissions are commonly subject to short term amplitude variations known as Rayleigh fading. For example, in a 1900 MHz system, a mobile station operating in a car traveling at 100 kilometers per hour experiences, on average, fading dips at a frequency of approximately 150 Hz. Since each timeslot is approximately 6.67 msecs in duration, which corresponds to a frequency of approximately 150 MHz, at least one fading dip can be expected during each timeslot.
While the transmit power level associated with one downlink burst may be more than sufficient to insure adequate signal quality, even if the signal is affected by a fading dip, the transmit power level associated with another downlink burst, particularly one being transmitted to a mobile station operating at or near the cell border, may not be sufficient to insure adequate signal quality if the signal is affected by a fading dip. For instance, if the signal associated with a given downlink burst is adversely affected by fading, the power level of the received signal may drop significantly, such that, the signal is extremely susceptible to co-channel interference, and to a lesser extent, adjacent channel interference, which may, in turn, render the quality of the signal unacceptable. Accordingly, an effective power control technique for TDMA based telecommunications systems is needed, whereby the power level of a given communications signal can be adjusted or modified during the course of a single timeslot so as to mask or reduce interference.
The present invention provides an effective power level control technique for use in TDMA based telecommunications systems, whereby the transmit power associated with certain downlink bursts, particularly those which are being transmitted with more power than the corresponding mobile station requires, is adjusted during a given timeslot, and in such a way, that the transmit power adjustment resembles that of a typical fading event, in terms of time of occurrence and rate of occurrence (e.g., dB per msec). In so doing, other mobile stations, using the same frequency carrier or an adjacent frequency carrier, receiving a downlink burst during that timeslot at a power level that is marginally adequate, such as mobile stations operating at or near the border of nearby cells, are better able to cope with the effects of fading, since they are subjected to less interference.
Thus, it is an object of the present invention to provide an effective power level control technique for use in TDMA based telecommunications systems.
It is another object of the present invention to provide a power level control technique that reduces interference.
It is still another object of the present invention to provide a power level control technique whereby mobile stations most susceptible to interference are not exposed to output power level changes during the timeslot in which they are receiving a downlink burst.
It is yet another object of the present invention to provide a power level control technique that improves the spectral efficiency and system capacity.
It is another object of the present invention to attenuate the power level associated with certain mobile stations during a corresponding timeslot in such a way that the mobile stations subjected to the power level attenuations are affected as little as possible.
It is another object of the present invention to attenuate the power level associated with certain mobile stations during a corresponding timeslot in such a way that the transmission quality for these mobile stations is maintained, while minimizing interference for mobile stations operating in nearby cells.
In accordance with one aspect of the present invention, the above-identified and other objects are achieved by a downlink power control method. The method involves transmitting a first downlink signal, from a base station to a first mobile station, where the first downlink signal is transmitted over a first frequency carrier and transmitted at a constant power level during a corresponding timeslot. In addition, a second downlink signal is transmitted to a second mobile station, where the second downlink signal is transmitted at an initial power level during the corresponding timeslot. The power level associated with the second downlink signal is then attenuated, wherein the attenuation of the power level associated with the second downlink signal reduces the likelihood that the second downlink signal will interfere with the first downlink signal, and wherein a signal quality associated with the second downlink signal is less likely to become unacceptable as compared to a signal quality associated with the first downlink signal, due to an attenuation in power level.
In accordance with another aspect of the present invention, the above-identified and other objects are achieved by a downlink power control method and/or apparatus. The method and/or apparatus involves transmitting a first downlink signal, from a base station to a first mobile station, where the first downlink signal is transmitted over a first frequency carrier and transmitted at a constant power level during a corresponding timeslot. A second downlink signal is then transmitted to a second mobile station, where the second downlink signal is transmitted at an initial power level during the corresponding timeslot. The power level associated with the second downlink signal is then attenuated in such a way that the attenuation of the power level associated with the second downlink signal resembles a typical fading event, where the attenuation of the power level associated with the second downlink signal reduces the likelihood that the second downlink signal will interfere with the first downlink signal.
In accordance with still another aspect of the present invention, the above-identified and other objects are achieved by a downlink power control method. This method involves transmitting from a base station to a corresponding mobile station, over a first frequency carrier, each of a number of downlink bursts at a corresponding power level during a respective timeslot. Next, a determination is made as to whether the transmit power level associated with a first downlink burst sufficiently meets the power requirements of the corresponding mobile station in the presence of fading. The transmit power associated with the first downlink burst is maintained at a constant level throughout a substantial portion of the respective timeslot, if it is determined that the transmit power level associated with the first downlink burst does not sufficiently meet the power requirements of the corresponding mobile station in the presence of fading, and the transmit power associated with the first downlink burst is attenuated during the respective timeslot, based on values associated with a typical fading event, if it is determined that the transmit power level associated with the first downlink burst sufficiently meets the power requirements of the corresponding mobile station in the presence of fading.
In accordance with still another aspect of the present invention, the above-identified and other objects are achieved by a downlink power control method that is employed by a time division multiple access based cellular radio telecommunications system, in which the time domain is divided into a plurality of time frames, and each time frame is divided into a plurality of timeslots, where each timeslot begins with a synchronization period, and where the radio telecommunications system includes a base station in radio communication with a first mobile station and a second mobile station on first and second timeslots of a carrier frequency, respectively. The method involves extracting values associated with the characteristics of a typical fading event and storing the extracted values in the base station. Next, it is determined that the second mobile station is requires a lower power level than the first mobile station. Then based on this determination, the transmit power level in the second timeslot is attenuated based on the stored values.